Security 8-73

0 AND 0 = 0

0 AND 1 = 0

1 AND 0 = 0

1 AND 1 = 1

For example:

Filter rule:

Deny

IP: 163.176.1.15BINARY: 10100011.10110000.00000001.00001111

Mask: 255.255.255.255BINARY:11111111.11111111.11111111.11111111

Incoming Packet:

IP 163.176.1.15BINARY: 10100011.10110000.00000001.00001111

If you put the incoming packet and subnet mask together with AND, the result is:

10100011.10110000.00000001.00001111

which matches the IP address in the filter rule and the packet is denied.

Implied rules

With a given set of filter rules, there is an Implied rule that may or may not be shown to the user. The implied rule tells the filter set what to do with a packet that does not match any of the filter rules. An example of implied rules is as follows:

Implied

Meaning

 

 

Y+Y+Y=N

If all filter rules are YES, the implied rule is NO.

 

 

N+N+N=Y

If all filter rules are NO, the implied rule is YES.

 

 

Y+N+Y=N

If a mix of YES and NO filters, the implied rule is NO.

 

 

Established connections

The TCP header contains one bit called the ACK bit (or TCP Ack bit). This ACK bit appears only with TCP, not UDP. The ACK bit is part of the TCP mechanism that guaranteed the delivery of data. The ACK bit is set whenever one side of a connection has received data from the other side. Only the first TCP packet will not have the ACK bit set; once the TCP connection is in place, the remainder of the TCP packets with have the ACK bit set.

The ACK bit is helpful for filter design and reduces the number of potential filter rules. A filter rule could be created just allowing incoming TCP packets with the ACK bit set, since these packets had to be originated from the local network.

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Netopia D7100 SDSL, D7171 SDSL, D3100-I IDSL, D3232 IDSL manual Implied rules, Established connections
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D7100 SDSL, D3232 IDSL, D3100-I IDSL, D7171 SDSL specifications

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